1. Field of the Invention
The present invention relates to an improved process for the hydroformylation of olefins in the presence of unmodified cobalt carbonyl complexes, in which the concentration of formic acid in aqueous solutions containing cobalt compounds is reduced by catalytic decomposition.
2. Discussion of the Background
Higher alcohols, in particular those having from 4 to 25 carbon atoms, may be prepared by the catalytic hydroformylation (or oxo reaction) of olefins having one less carbon atom, followed by catalytic hydrogenation of the resulting aldehyde- and alcohol-containing reaction mixtures. These alcohols may be used, for example, in preparing plasticizers and detergents.
The type of catalyst system and the optimum reaction conditions for the hydroformylation depend on the reactivity of the olefin used. The dependence of the reactivity of the olefins on their structure is described, for example, by J. Falbe, New Syntheses with Carbon Monoxide, Springer-Verlag, Berlin, Heidelberg, N.Y., 1980, page 95 ff. In particular, the differing reactivity of isomeric octenes is known (B. L. Haymore, A van Hasselt, R. Beck, Annals of the New York Acad. Sci., 415 (1983), pages 159-175).
Industrial olefin mixtures which are used as starting materials for the oxo process comprise olefin isomers having a variety of structures, for example with different degrees of branching, different positions of the double bond in the molecule and possibly also different numbers of carbon atoms. This is particularly true of olefin mixtures which have been formed by dimerization or trimerization, or further oligomerization of C2-C5 olefins or other readily available higher olefins, or by cooligomerization of such olefins. For example, tripropenes and tetrapropenes and also dibutenes, tributenes and tetrabutenes are typical isomeric olefin mixtures which may be reacted by rhodium-catalyzed, or preferably cobalt-catalyzed hydroformylation reactions, to give the corresponding aldehyde and alcohol mixtures.
If alcohols having a very low degree of branching are desired, the hydroformylation reaction may preferably be carried out using unmodified cobalt catalysts. Compared to rhodium hydroformylation catalysts, cobalt hydroformylation catalysts provide higher yields of the particularly valuable oxo products having a higher content of straight chains.
The hydroformylation of olefins catalyzed by unmodified cobalt carbonyl complexes provides reaction mixtures comprising aldehydes, alcohols, their formic esters, unreacted olefins, free formic acid and additional by-products, as well as the catalyst. In order to obtain an almost cobalt-free reaction mixture suitable for further processing, the various cobalt compounds present in the hydroformylation product have to be removed. In addition, because the cobalt hydroformylation catalysts are expensive, any cobalt catalysts and catalysts residues which have been separated off should preferably be recycled.
During the hydroformylation and/or during removal of the cobalt compounds from the hydroformylation product, formic acid is formed by the hydrolysis of formic esters. The presence of formic acid inhibits the formation of active hydroformylation catalysts, for example, cobalt carbonyls, when the cobalt compounds of the hydroformylation product are regenerated to re-form active hydroformylation catalysts.
A conventional method for recovering the catalyst from the hydroformylation mixture comprises extracting cobalt compounds from the reactor output with an aqueous base, acidifying the extract to liberate HCo(CO)4, and returning the HCo(CO)4 to the hydroformylation reactor (Kuhlmann process). However, a disadvantage of this process is that one must dispose of the by-product salt of the base used to extract the cobalt compounds from the reactor output.
GB 2055371 describes a process comprising the following steps:
a) A portion of the cobalt carbonyls present in the hydroformylation product is reacted with an aqueous cobalt(II) salt solution to form Co{Co(CO)4}2. At the same time, the cobalt complex salt is extracted into the aqueous phase.
b) In a second step, the remaining cobalt carbonyls are oxidized with oxygen in the presence of an aqueous acid to give cobalt(II) salts. The aqueous extract thus formed is recycled to step a), above.
c) The active cobalt compounds, i.e. the cobalt hydroformylation catalyst, are prepared by reaction of the extract of step (a) with synthesis gas in the presence of an organic solvent.
A disadvantage of this process is that the separation of the cobalt compounds from the hydroformylation product, alone, requires an oxidation step and two phase separations.
WO 93/24437 describes a process for catalyst removal and regeneration, which essentially comprises the following steps:
a) After reducing the pressure of the hydroformylation output, the cobalt compounds are extracted with an aqueous solution having a pH of not more than 7, in particular an aqueous solution comprising formic acid, under either reductive conditions, or oxidative conditions (e.g., in the presence of oxygen).
If oxidative extraction conditions are employed, the following work-up procedure is used:
b) The aqueous extract comprising the cobalt compounds is concentrated by distillation. The top product (i.e., the lower density phase) is a mixture of water and acids (formic acid), part of which is recycled back to the extraction step a).
c) The aqueous concentrate from step b) is reacted with synthesis gas at elevated temperature at greater than atmospheric pressure to give a mixture comprising cobalt carbonyls.
d) The cobalt carbonyls are then stripped from the mixture obtained from step c). The remaining aqueous stream is recirculated back to step a).
e) The cobalt carbonyls are extracted from the stripping gas using the olefin starting material (for the hydroformylation).
In the nonoxidative work-up, the same process steps are carried out, but in a different order (a-d-c-b) and with the further difference that the largely cobalt-free aqueous stream from step d) is divided into two substreams which are recirculated to steps a) and b).
WO 93/24438 and U.S. Pat. No. 5,321,168 describe further developments of the process disclosed in WO 93/24437. In the same sequence of work-up steps, a palladium catalyst is additionally used for catalyst preformation (step c). In addition, U.S. Pat. No. 5,321,168 describes regenerating the activity of the palladium catalyst.
All of these work-up methods are very complex and incur high capital and operating costs.
An additional process is described in J. Falbe, New Syntheses with Carbon Monoxide, Springer-Verlag, Berlin, Heidelberg, N.Y., 1980, page 164, 165 (BASF process) and comprises the following steps:
a) Oxidation of the cobalt carbonyls by oxygen in the presence of acids, in particular formic acid formed in the process, to give cobalt(II) salts, followed by their extraction from the hydroformylation mixture.
b) Reduction of the cobalt(II) salts in the aqueous extract from step a) in the presence of an organic solvent to give cobalt carbonyls.
c) Extraction of the cobalt carbonyls into an organic phase, preferably the starting olefin, which is then fed into the hydroformylation reactor.
DE 196 54 340 describes a lower capital and operating cost hydroformylation process in which steps b) and c) are carried out in situ in the hydroformylation reactor. The space-time yields of this process also depend on the formation and stability of the catalyst complex in the hydroformylation reactor. The space-time yield may be further improved.
In all of the conventional processes described above, the formic acid concentration is established by the reaction conditions, and is generally neither influenced nor controlled at all, or is reduced by a further process step such as distillation.
It is therefore an object of the invention to develop a hydroformylation process for olefins which has a closed cobalt catalyst circuit which simplifies catalyst recirculation and provides for high space-time yields.
Applicants have surprisingly found that the space-time yield of a hydroformylation reaction can be increased if the formic acid concentration is reduced in either the aqueous cobalt(II) salt solution which is fed to the hydroformylation reactor, or in the over-all process.
The present invention accordingly provides a process for preparing aldehydes having from 4 to 25 carbon atoms by hydroformylation of the corresponding olefins having 3 to 24 carbon atoms in the presence of a cobalt catalyst, wherein the formic acid formed during the hydroformylation reaction, or in the subsequent catalyst work-up, is decomposed catalytically.
The decomposition of the formic acid is preferably carried out in an aqueous phase which is subsequently, i.e. after decomposition of the formic acid has occurred, returned in full or in part to the hydroformylation process.
The crude aldehydes prepared by the process of the present invention are preferably hydrogenated to form alcohols, which may be used in products such as plasticizers and detergents.
The hydroformylation of olefins and the separation of the cobalt catalyst from the hydroformylation product can be carried out, for example, as described in DE 196 54 340 A1. This is a single-stage process in which the olefins are reacted with an aqueous cobalt salt solution and synthesis gas in a homogeneous phase, preferably in a cascaded reactor, particularly preferably by means of a mixing nozzle. The reaction product is subsequently treated oxidatively in the presence of an aqueous solution of carboxylic acids, predominantly formic acid. After phase separation, part of the aqueous phase comprising the cobalt salts is returned to the single-stage process and the organic phase is further treated to give the products (e.g., aldehydes) or hydrogenated to the corresponding alcohols. The aqueous solution recirculated to the hydroformylation reactor contains virtually the same amount of cobalt as is discharged with the hydroformylation mixture.
In principle, it is possible to catalytically decompose the formic acid in the hydroformylation reaction mixture, either before or after oxidation of the cobalt carbonyls. The cobalt compounds may then be separated from the hydroformylation product, afterward.
The process of the invention is preferably carried out by separating the components of the hydroformylation reaction mixture in the presence of an aqueous phase, by oxidative treatment of the hydroformylation reaction mixture and subsequent phase separation of the mixture into an organic product phase and an aqueous phase comprising cobalt salts and formic acid. All or part of the aqueous phase may be treated to catalytically decompose the formic acid. Any portion of aqueous phase in which the formic acid is not decomposed may be recycled back to the cobalt removal step. The largely formic acid free aqueous solution thus obtained can be recirculated back to the hydroformylation process.
In order to reduce the concentration of formic acid in the over all process, it may be sufficient to recycle back to the hydroformylation reactor only those portions of the aqueous phase in which the formic acid has been catalytically decomposed. Alternatively, it may be possible to reduce the concentration of formic acid in the over all process by decomposing only part of the formic acid in the decomposition reactor. In most cases, it is sufficient for at least 50%, preferably from 50 to 80%, of the formic acid formed in the process to be decomposed catalytically.
In any case, the solution or mixture which is treated to decompose the formic acid contains cobalt compounds, either as cobalt carbonyl or, after the oxidative work-up, as a cobalt(II) salt. The concentration of these cobalt compounds may vary within a wide range, as long as no cobalt compounds precipitate during the work-up. The concentration of the cobalt salt solution is partly determined by the amount of water which is discharged from the reactor with the hydroformylation mixture. Since water is discharged from the process with the essentially cobalt-free hydroformylation mixture (i.e. the organic phase after the catalyst has been separated off) it may be necessary to add water in order to avoid an increase in the cobalt compound concentration. The water may be added, for example, to the reactor feed, the cobalt removal unit or the crude product scrubber.
If desired, part of the formic acid may be separated off by distillation, and the formic acid in the distillate may be decomposed by the process of the present invention.
It is known that formic acid reacts with oxygen in the presence of heavy metal catalysts, in particular Pd catalysts, to form carbon dioxide and water. Likewise, formic acid can be oxidized in the absence of catalysts, e.g. using hydrogen peroxide.
Suitable formic acid decomposition catalysts used in the process of the present invention may comprise metals of groups VIII, VIIa and Ib of the Periodic Table of the Elements. These metals may be used in the form of the elemental metal or as metal oxides, in each case as a mixture of metals or as a mixed oxide. The catalyst may be supported on support materials including, for example, Al2O3, SiO2, TiO2, MgO and their mixed oxides, zeolites or activated carbon. In the process of the invention, supported catalysts, in particular Pd/Al2O3 catalysts, are preferred.
During the oxidative removal of formic acid in aqueous cobalt salt solutions, cobalt(II) is partly oxidized to cobalt(III), which may lead to precipitation of Co(III) compounds as the pH increases. When the decomposition of formic acid is carried out in the absence of oxygen or another oxidant, this reaction does not take place. For this reason, nonoxidative methods for decomposing formic acid are preferred, e.g. decomposition of the formic acid in the presence of synthesis gas, hydrogen, carbon monoxide, carbon dioxide or nitrogen.
In particular, the formic acid in aqueous cobalt salt solutions produced by the oxidative removal of cobalt from the hydroformylation product, can be decomposed catalytically.
The process of the present invention, in which formic acid is catalytically decomposed together with hydroformylation, may be carried out continuously or in a batch process, and either in the liquid phase or at least partly in the gas phase. Continuous decomposition in the liquid phase is the preferred method of carrying out the process.
The decomposition of formic acid can be carried out in stirred reactors or preferably in tube reactors. In tube reactors, the decomposition of formic acid can be carried out in a single pass or with recirculation of the solution (loop). The reactors may be operated as co-current reactors with trickle flow or with high liquid throughputs (pulse flow). It is also possible to connect a plurality of reactors together with one another. Each of these reactors may be operated in the same mode or in different modes.
The decomposition of formic acid is carried out in a temperature range from 100xc2x0 C. to 300xc2x0 C., preferably in the range from 120xc2x0 C. to 200xc2x0 C. The total pressure is generally from 1 bar to 350 bar, in particular from 1 bar to 25 bar.
The space velocities over the catalyst during the formic acid decomposition are from 0.5 hxe2x88x921 to 5 hxe2x88x921, in particular from 1 hxe2x88x921 to 3 hxe2x88x921.
If desired, the formic acid decomposition can be carried out in the presence of a purge gas, for example carbon monoxide, nitrogen, hydrogen or synthesis gas.
After concentration and premixing with starting material and possibly with synthesis gas, the aqueous solutions in which the concentration of formic acid has been reduced or eliminated entirely may be fed back into the hydroformylation reactor.
In another embodiment of the present invention, the catalytic decomposition of the formic acid is carried out in the presence of synthesis gas to form the cobalt catalyst used in the hydroformylation of the olefin starting materials (known as preformation).
The preparation of this cobalt catalyst (HCo(CO)4 and Co2(CO)8) by reacting aqueous cobalt(II) salt solutions with synthesis gas at elevated pressures and temperatures is, as described above, known. This reaction may be accelerated by means of Pd catalysts, as is described in the patents U.S. Pat. No. 5,321,168, U.S. Pat. No. 5,434,318 and WO 93/24438. In the Pd catalyzed reactions, an aqueous cobalt(II) salt solution is reacted with synthesis gas over a Pd catalyst in the presence of an organic solvent, preferably an alcohol. A disadvantage of these methods is that the catalyst is deactivated and has to be periodically regenerated. The regeneration may be carried out as described, for example, in U.S. Pat. No. 5,321,168 by scrubbing the deactivated catalyst with water, with or without addition of formic acid, in the presence of synthesis gas at a pressure of 138-310 bar and a temperature of 120-170xc2x0 C. According to the method described in U.S. Pat. No. 5,434,318, scrubbing is carried out under the above-mentioned conditions using a mixture of alcohol and aqueous formic acid.
The processes described above encompass only the formation of cobalt carbonyls from cobalt(II) salt solutions, but not the catalytic decomposition of formic acid.
In a particular embodiment of the present invention, the decomposition of formic acid can be carried out with simultaneous formation of cobalt carbonyl compounds, as either a batch or continuous process.
The combined reactions may be carried out in stirred reactors or preferably in tube reactors. If tube reactors are used, the reaction may be carried out in a single pass or with recirculation of the solution (loop). The tube reactors may be operated as co-current reactors with trickle flow or with high liquid throughputs (pulse flow). It is also possible for a plurality of reactors to be connected to one another. These reactors may be operated in the same mode or in different modes. The combined reactions may be carried out in the presence of synthesis gas at a total pressure of from 150 bar to 330 bar, in particular from 200 bar to 300 bar.
The molar ratio of carbon monoxide to hydrogen in the synthesis gas used in the combined catalyst synthesis and regeneration reactions can be in the range of from {fraction (2/1)} to xc2xd.
The reaction temperatures in the combined formic acid decomposition and preformation reactions may be in the range of from 140xc2x0 C. to 220xc2x0 C., preferably from 160xc2x0 C. to 180xc2x0 C. The space velocities over the catalyst (LHSV) are from 0.5 hxe2x88x921 to 5 hxe2x88x921, preferably from 1 hxe2x88x921 to 3 hxe2x88x921.
The same catalysts either for the decomposition of the formic acid alone, or for simultaneous decomposition of formic acid and formation of the cobalt catalyst.
The liquid reaction product from the combined decomposition and preformation reactor is introduced into the hydroformylation reactor. The waste gas obtained may be fed to the oxidizer, or preferably may be fed to the hydroformylation reactor.
If desired, the decomposition of the formic acid, together, if desired, with simultaneous formation of cobalt carbonyls, may be carried out in the presence of organic solvents comprising, for example, alcohols or aldehydes, in particular the starting materials and products of the hydroformylation reaction. These are preferably liquid hydrocarbons and/or the corresponding olefins which are liquid under the reaction conditions of the hydroformylation.
In any case, it is possible to recirculate the cobalt catalyst formed in the decomposition of formic acid to the hydroformylation reaction.
A hydroformylation plant operating according to the process of the present invention requires a somewhat higher capital investment compared to a conventional hydroformylation plant. This small capital cost disadvantage is more than compensated for by the higher space-time yield of the process of the present invention and also further advantages:
The formic acid concentration in the overall plant, in particular in the lower part of the hydroformylation reactor and in the downstream oxidizer (cobalt removal unit), is reduced.
The hydroformylation rate and thus the space-time yield of the hydroformylation reaction is increased, since the mean concentration of active cobalt compounds in the hydroformylation reactor is greater.
A further advantage is that the decomposition of the major part of the formic acid generated in the process results in the production of waste streams containing less formic acid. This reduces environmental pollution.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.